Oxidation resistant iron base alloy

ABSTRACT

IRON BASE ALLOYS WITH ALLOYING ADDITIONS OF GROUPS III AND IV ELEMENTS AND/OR BERYLLIUM ARE DESCRIBED WHICH EXHIBIT UNIQUE AND UNEXPECTEDLY LOW WEIGHT GAINS WHEN EXPOSED TO OXIDIZING ENVIRONMENTS. ONE OF THE MAJOR ADVANTAGES OF THE GROUP OF ALLOYS DESCRIBED HEREIN LIES IN THEIR MODERATE COST, ESPECIALLY AS COMPARED TO OTHER OXIDATION RESISTANT FE-BASE ALLOYS SUCH AS THE VARIOUS GRADES OF STAINLESS STEEL. THE OXIDATION RESISTANCE IS IMPARTED TO THESE ALLOYS THROUGH THE FORMATION OF A TENACIOUS COMPACT OXIDE FILM CONSISTING ESSENTIALLY OF ONE OR BOTH OF THE ADDED ALLOYING ELEMENTS.

May 1, 1973 Filed Aug. 3;), 1970 E.J.CAULE ETAL 3,730,779

OXIDATION RESISTANT IRON BASE ALLOY 5 Sheets-Sheet l 700 aoo 400 TEMPERATURE INVENTORS v ELMERJ. CAULE' MICHAEL J. PRYOR JAMES-A. FORD ATTORNEY OXIDATION RESISTANT I-ROX EASY-' ALLOY Filed Aug. 25, 1970 5 Sheets-Sheet 2 TEMPERATURE ch Q Q w YINVENTORS PHIL/P R. SPERRY MICHAEL J PRYOR JAMES A. FORD ELMERJ. CAULE May 1, 1973 CAULE ETAL 3,730,779

OXIDATION RESlJTANT IRON BASE ALLOY v Fi1 0dAl1g. 1973' 5 Sheets-Sheet i gs U q k 3 5 3, k g m E k s Q 2 &

v N I k b 0 & Q

0 CD Tm I L i5 0% INVENTORS ELMER JCAULE PHILIP R. SPERRY MICHAEL JPRYOR JAMES A. FORD ATTORNEY May 1, 1973 E,J,CAU1 E ET AL 3,730,779

I OXIDATION RESISTANT IRON BASE ALLOY Filed Aug. 25, 1970 5 Sheets-Sheet 4 400 500 TEMPERATURE "6 ATTORNEY May 1,1973

Filed Aug. 25, 1970 E. J. CAULE ET AL 3,730,779

OXIDATION RESISTANT IRON BASE ALLOY 5 Sheets-$heet 5 T- 301 STAINLESS 0 8 5 2,000 E i E k 100 200 300 400 500 600 700 I R Y O TEMPERATURE C 0 v INVENTORS ELMER J. CAULE' r 5 PHIL/P R SPERRV MICHAEL J. PR YOR JAMES A FORD United States Patent Oflice 3,730,779 OXIDATION RESISTANT IRON BASE ALLOY Elmer J. Caule, 440 Yale Ave., New Haven, Conn. 06515; Philip R. Sperry, 60 Pool Road, North Haven, Conn. 06473; Michael J. Pryor, Maplevale Drive, Woodbridge, Conn. Pool Road, North Haven, Conn. 06473 Original application Aug. 7, 1968, Ser. No. 750,991, now abandoned. Divided and this application Aug. 25, 1970,

Ser. No. 66,866

Int. Cl. C23f 7/04 US. Cl. 148-655 2 Claims ABSTRACT OF THE DISCLOSURE This application is a division of copending application Ser. No. 750,991, filed Aug. 7, 1968, now abandoned.

It has long been an objective of the art to achieve a low cost Fe-base alloy with oxidation resistance superior to that exhibited by the stainless steels. Presently, the only known method for providing such oxidation resistance relies in the formation of an adherent Cr surface layer. This requires large amounts of Cr in solid solution; in fact, it has been shown that a minimum of about 11 weight percent Cr is necessary to provide the very excellent oxidation resistance characteristics of stainless steels.

A class of alloys is described in this invention which are essentially low alloys of Fe and which have oxidation resistance significantly greater than achieved by stainless steels in the temperature range from 300 through 1000 C.

This unusual oxidation resistance is achieved through the addition of combinations of Group III and Group IV alloying elements. Be may be used instead of, or in addition to, Group III elements. Specifically, extensive research has shown that the combination of -Al and Si in a high purity Fe-base provides unusual and unexpected oxidation behavior in a high temperature air environment. It has been shown that over the composition range of 1-7% Al and 1-4% Si oxidation weight gain of the alloys of this invention is on the order of 50 times less than that experienced by stainless steels or other Fe-base alloys.

As shown in FIG. 1, the combination of 3% Al and 2% Si produces a unique and unusual reversal in oxidation weight gain in the temperature range of 600 to 900 C. This is in distinct contrast to the oxidation weight gain exhibited by stainless steel (type 30ll7Cr-7Ni) shown in FIG. 2 over the same temperature range.

The oxide films produced in this temperature range are nearly colorless and adherent. On the other hand,

certain (type 301) stainless steels begin to form nonadherent scales at the higher temperatures, i.e., 900 C. We have shown that this unique and distinct reversal in oxidation weight gain occurs over the range of chemistry given above for the Al-Si system.

FIG. 3 is a ternary plot of the iron-aluminum-silicon alloys of the present invention showing weight gains in micrograms per square centimeter after the alloys having the composition shown were heated for two hours at 06515; and James A. Ford, 51'

3,730,779 Patented May 1, 1973 800 C. FIG. 3 also shows by comparison the weight gains of binary iron-silicon alloys and binary ironaluminum alloys. It is apparent that the alloys tested within the range of 1-3% silicon and 24% aluminum had much lower Weight gains than the binary alloys. In fact, from this data, it is apparent that weight gains below 50 micrograms per square centimeter are assured for the two-hour treatment at 800 C.

While the examples of alloys of this invention discussed so far were carried out with Al-Si alloys, the present invention is applicable to Be and other Group 111(k) and Group IV(b) alloying elements. Specifically, combinations of Be-Si, Gallium-Si, Al-Germanium. The following compositions may be used, for example, 3% Be-2% Si; 3% In-2% Si; 3% Ga2% Si; 3% Al-2% Ge. Alloys of this group are superior to stainless steels in weight gain at elevated temperatures. For example, at temperatures above 725 C., up to 1000 C. and higher, the Ga-Si alloy weight gain was much less than many types of stainless steel, as shown in FIG. 4.

The unexpected reversal in the plot of weight gain against temperature is believed to result from the formation of A1 0 and a complex of the type of FeO.Al O in the case where aluminum is one of the Group UK b) elements. Or, if gallium is the Group II'I(b) element, it would be Ga O and a complex of FeO.Ga O The Group IV (b) elements, such as silicon and germanium, are believed to stimulate the formation of oxides of this type.

It will be apparent that the alloys of the present invention are considerably less expensive than alloys containing chromium in amounts of at least 5 and generally above 11; for example, 18 Cr-8 Ni is commonly used.

The Fe-base alloys of the present invention may be processed according to ordinary Fe-base melting, casting and hot and cold work procedures. The alloys of the present invention do not require specialized equipment or technology.

For example, the alloys of the present invention may be vacuum cast, homogenized, for example, at 1800 F., hot rolled to approximately 0.200 inch, with a reheat to 1800 R, if desired, scalped, and cold rolled without interannealing to from 0.200 gage to 0.015.

The steel to be used in the present invention does not, however, require vacuum casting. Ordinary steel making procedures may be used, such as blast furnace treatment with either open hearth or LD process reduction of the carbon content, as is well known in the art. The resulting composition of the alloys may be as follows: phosphorus and sulfur, 0.04 max. each, manganese may be up to 1.5%, preferably up to 0.2%, copper may be present as high as 2%, but preferably not more than 0.5%, others, including nickel, molybdenum, vanadium, and other alloying elements commonly present in low alloy steels may be present up to 0.20%. The carbon content may range as high as 2%, for some applications requiring very high wear resistance. However, the carbon content is preferably not more than about 1%. Furthermore, the most preferred range of carbon is from 0.01 to 0.25%. It is expected that the latter range will have the most general application.

As a further superior feature of these alloys, it has been found that the alloys of the present invention are highly resistant to atmospheric oxidation and tarnishing after first being exposed to ahigh temperature air oxidation for 5 minutes to hours at 400 to 1000 C., preferably 600 to 1000 C. For example, two hours at 800 C. has been found to be very satisfactory.

It has also been found that these alloys are resistant to contaminated atmospheres containing sulfur and/ or water vapor and/or carbon monoxide-dioxide combinations.

These atmospheres are representative of most hydrocar bon combustion gases.

FIG. 5 compares behavior of a 3.0% Al-2.0% Si alloy with type 301 stainless steel in a wet oxidizing S0 atmosphere. Above 700 C., the Al-Si alloy is clearly superior.

It is to be understood that the invention is not limited to the illustrations described and shown herein which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modifications of form, size, arrangement of parts and detail of operation, but rather is intended to encompass all such modifications which are within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A process which comprises: providing an alloy consisting of (A) an alloying addition in the amount of from 1 to 7% selected from the group consisting of beryllium, aluminum, indium and gallium and (B) an alloying addition in the amount of from 1 to 4% selected from the group consisting of silicon and germanium, phosphorus 0.04% max., sulfur 0.04% max., manganese up to 0.2%, copper up to 0.5%, carbon up to 2%, others up to 0.20%, balance iron; exposing said alloy to oxidation at a temperature of from 300 to 1000 C. for a period of time of from 5 minutes to 100 hours, thereby forming a substantially colorless and adherent oxide film containing a complex oxide having oxidation and tarnish resistance.

2. A process according to claim 1 including the step of exposing said alloy to a contaminated atmosphere.

References Cited UNITED STATES PATENTS 1,759,605 5/1930 De Vries *-124 1,781,527 11/1930 Saklatwalla 75-124 3,377,213 4/ 1968 Hiller 148-635 3,206,304 9/ 19'65 'Bechtold 75124 RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 75-124 

